In an interview with R.L. Stine, he revealed that he is working in a NEW Goosebumps SlappyWorld book titled I Am Slappy's Evil Twin. This information was revealed in Streamable. 

The link is here:

This book presents a "evil twin" for Slappy the Dummy? A new enemy? This book is a poorly parody from I Am Your Evil Twin? Mr. Wood return? I'm so excited that released the book! :D and you??

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